Gas water heater

ABSTRACT

A domestic gas storage water heater utilizes a plastic non pressure tank to deliver mains pressure hot water. The heater may be external or internal of the tank and has a heat out put of about 4-10 Mega joules per hour. A pressure transfer module uses the mains pressure of the incoming cold water to deliver hot water from the tank at mains pressure. The tank may be of larger storage capacity than conventional gas heaters and the arrangement ensures that the heated water in the tank is stratified into layers of different temperature. This enables an uninterrupted supply of hot water to be achieved.

BACKGROUND TO INVENTION

[0001] Gas water heaters that are commercially available and delivermains pressure water are characterized as having a stainless steel tankor a vitreous enamel lined mild steel tank with a high rating gasburner. These heaters don't have a large capacity water storage but canheat the water quickly.

[0002] The heat efficiency of such heaters is not high and the output ofCarbon dioxide, a green house gas, is quite high. Usually the heatexchange is achieved by passing the hot combustion gases up a fluethrough the centre of the tank or over the external surface of the tank.Australian patent 720062 is an example of this approach. This has thedisadvantage of having to heat the total volume of water in the storagetank to the desired temperature.

[0003] U.S. Pat. No. 4,503,810 discloses a combustion gas water heaterin which the hot combustion gases do not exchange heat with the storedwater but rather a small volume of water is quickly heated in directexchange with the combustion gases in an external heat exchanger. Coldwater is drawn from the bottom of the storage tank heated in theexternal heat exchanger and then returned to the upper or middle portionof the storage tank by convection.

[0004] Another problem with water heater storage tanks is corrosion ofthe tank which occurs with unlined and also vitreous enamel lined tanks.The use of non-corrodible stainless steel tanks is an expensive solutionto the problem. Plastic tanks are cheaper but these are unsuitable forstoring mains pressure hot water.

[0005] One proposal has been to use plastic lined metal tanks asproposed in U.S. Pat. No. 4,338,888. The combustion chamber has waterwalls for heat exchange. Water from the bottom of the tank is heated inthe water walled combustion chamber and the heated water is returned tothe lower portion of the tank.

[0006] Another approach for electric heated tanks is USA patent 4437484which uses an un-pressurised plastic tank and a pump.

[0007] It is an object of this invention to provide a storage waterheater which overcomes these problems.

BRIEF DESCRIPTION OF THE INVENTION

[0008] To this end the present invention provides a combustion waterheater which includes

[0009] a) a non-pressurised water storage tank having a cold water inletin the lower portion of said tank and a heated water outlet in the upperportion of said tank

[0010] b) a low heat output combustion chamber having a flue for theegress of combustion gases

[0011] c) a heat exchange surface associated with said flue

[0012] d) means for contacting water from the lower portion of saidwater storage tank with said heat exchange surface

[0013] e) means for conducting water heated at the heat exchange surfaceto the upper portion of said water storage tank

[0014] f) pump means associated with the heated water outlet to increasethe pressure of water from the non pressurised tank.

[0015] By using a non-pressurised tank a plastic tank can be utilisedwith the resultant advantages of longer service life due to lesscorrosion. By returning the heated water from the heat exchanger to theupper portion of the tank, temperature stratification of the water inthe tank is established. This has the benefit that water extracted fromthe upper portion of the tank is always the hottest water in the tank.The plastic used can be any structural polymer that is heat resistantand suitable for the application. Stratification of heated layers ofwater is achieved by introducing the hot water to the top portion of thetank and separating any hotwater being heated in the bottom of the tankfrom cold water by using a separate chamber or a shroud.

[0016] The use of a low joule burner in combination with a heatstratified tank means that the stored water can be cost effectivelyheated over a longer period of time. Because the hottest layers areadjacent the outlet there is little risk that the supply of hot waterwill be interrupted. The low joule burner is preferably rated at 1 to 2kw.

[0017] The pump adjacent the outlet ensures the that the water pressurefrom the hot water taps will be at a pressure equivalent to mainspressure. Preferably the pump is a multi compartment pressure transfermodule that utilises the mains particularly high density polypropylene.The tank can be formed by any suitable moulding technique although forhigh density poly propylene blow moulding is preferred. The storagevolume of the tank may be greater than that which is conventional forgas heaters but equivalent to that of electric storage heaters intendedto supply similar daily volumes. A domestic size tank would be 315litres storage capacity but smaller or larger sizes are feasible. Alarge tank is required when the power rating of the combustion chamberis selected to provide a time to heat the whole tank of about 12 to 20hours. The dimensions of such a tank are typically about 660 mm diameterand 1700 mm height. For smaller tanks a higher rating burner could beused to provide a larger proportion of the tank with hot water.

[0018] The tank 1 is provided with an outer skin 2 and a layer 3 ofinsulation to limit heat losses from the tank. The outer skin ispreferably of a hard wearing, tear resistant and weather resistantplastic such as high density polyethylene, low density polyethylene,ABA, ABS or even sheet metal. Sheet metal has the advantage of havinglower thermal expansion characteristics and may also be less expensive.The insulation material can be any suitable material with low heatconductivity with polyurethane foam being the preferred material.

[0019] The removable lid 4 also has an insulation layer between an innerand outer skin. Because the tank is not intended to operate at mainswater pressure the seal between the tank and the lid need not beparticularly strong. This means that access to the interior of the tankfor manufacturing and maintenance purposes is much easier than for mainspressure tanks. The lid 4 is arranged to seat on the tank above theoperating water level 9 of the tank.

[0020] The pressure transfer module 5 is of the type described inWO97/46805. The contents of that patent are incorporated herein byreference. Cold water passes into the module through mains pressure coldwater inlet 10 and after being reduced in pressure the water flowsthrough cold water outlet 12 into the lower portion of the tank.

[0021] The PTM is either positioned in the top of the tank or draws hotwater from the top layer of water in the tank. Hot water is drawn intothe PTM 5 and exits at mains pressure through the hot water outlet 11.

[0022] In the embodiment shown in FIG. 1 cold water from the bottom ofthe tank is drawn into the heat exchanger 7 and passes up the heatedwater delivery pipe into the upper section of the tank. The externalheat exchanger 7 comprises a gas flame 6 mounted in a chamber with anassociated air inlet. The hot combustion gases are caused to circulatearound pipes or water walls in the heat exchanger and then to exit viathe flue outlet 8. In FIG. 1 the flue is shown as balanced with the flueoutlet 8 being adjacent the air inlet for gas flame 6.

[0023] The embodiment of FIG. 2 is a first version of a tank with thegas flame 6 and heat exchanger 7 disposed in the lower portion of thetank. A shroud or convection flume 13 confines the cold water enteringthe bottom of the tank through inlet 12 so that it is in heat exchangerelation ship with the external surface of the heat exchanger 7. Thearrangement of heat exchanger 7 within the shroud 13 may be inaccordance with the well known principals of heat exchangers inincreasing the heat transfer surface area without impeding the flow offlue gases from the flame 6 to the flue outlet 8. In FIG. 2 the flueoutlet 8 is disposed in the lid 4 of the tank. To maximise the heattransfer the hot water delivery pipe 14 surrounds the vertical flue 16up into the upper portion of the tank to terminate just below the waterlevel 9.

[0024] In the embodiment of FIG. 3 which is a variation of theembodiment of FIG. 2 the flue is balanced with the flue outlet 8 beingadjacent the air inlet for flame 6. As shown in FIG. 3 the heatexchanger 7 provides a path for the flue gases which traverses the baseof the tank and then doubles back . The arrangement of heat exchanger 7can be designed to provide maximum heat transfer area between theconduits for the combustion gases and the water contained within shroud13. In the embodiment of FIG. 3 the hot water delivery tube 14 can be ofsmaller diameter.

[0025] In all three embodiments the operation of the gas flame may befan assisted to ensure complete combustion of the fuel and to assist inthe positive flow of the flue gases to the outlet 8. In the embodimentof FIGS. 2 and 3 the fan and gas mixing valve may be situated on the topof the tank to avoid having them attached to the external wall of theunit which would increase the floor imprint of the unit. This has theadditional advantage of keeping the fan away from dust accumulation nearthe bottom of the unit. The fan and gas valve can be a unit in which thespeed of the fan controls the gas inlet valve to maintain the correctair/gas mixture required for clean combustion. A pre-mix burner with aDC fan is preferred. In a simple unit with on temperature sensor a onespeed fan would be suitable. A two speed fan would need two temperaturesensors one higher than the other.

[0026] The burner used with a 315 litre tank in accordance with any ofthe above 3 embodiments is rated at 5-6 Mega-joules/hour which given theefficiency of the stratification in the tank and the insulation on thetank gives a heating out put of about 1.1-1.3 Kw. The efficiency ratingis about 80%. The burner may be a simple flame into the heat exchangeror may be a flame supported on a thimble like mesh shroud or even alonger mesh burner to extend within the tank for the embodiments ofFIGS. 2 and 3.

[0027] The burner can be controlled by a control unit which includes oneor more temperature sensors within the tank. With one sensor placed nearthe bottom of the tank a temperature above the set temperature for thetank would trigger the switching off of the burner. With two sensors onenear the top and one near the bottom variable heating rates can beutilised. A low temperature at the upper sensor would trigger a higherinput rate for the fan to increase the heat out put until the uppersensor exceeded the set temperature.

[0028] For domestic dwellings, the water heater of this inventionprovides an uninterrupted supply of mains pressure hot water at lowenergy cost and low green house emissions. In addition the tank has alonger useful life than conventional gas water heaters of equivalentcapital cost.

1. a combustion water heater which includes a) a non-pressurised waterstorage tank having a cold water inlet in the lower portion of said tankand a heated water outlet in the upper portion of said tank b) a lowheat output combustion chamber having a flue for the egress ofcombustion gases c) a heat exchange surface associated with said flue d)means for contacting water from the lower portion of said water storagetank with said heat exchange surface e) means for conducting waterheated at the heat exchange surface to the upper portion of said waterstorage tank f) pump means associated with the heated water outlet toincrease the pressure of water exiting from the non pressurised tank. 2.A water heater as claimed in claim 1 in which the water in the tank isstratified by temperature with the hottest water adjacent the hot wateroutlet.
 3. A water heater as claimed in claim 1 wherein the heatexchange surface is disposed within the water storage tank.
 4. A waterheater as claimed in claim one wherein the pump means is a multicompartmented pressure transfer module adapted to release hot water atnear mains pressure by utilising the existing mains pressure of the coldwater inlet.
 5. A water heater as claimed in claim 1 wherein the heatexchange surface is disposed within the lower portion of the tank andthe tank incorporates a shroud which separates the water in the lowerportion of the tank, which surrounds the heat exchanger, from the waterin the remainder of the tank, and a water outlet allows water fromwithin the lower portion to rise to the upper section of the tank.
 6. Awater heater as claimed in claim 1 or 2 in which the energy out put ofthe combustion unit is 1 to 2 kilowatts.